The formation of giant podiform chromitite by asthenospheric melts in supra-subduction zone environments

Podiform chromitite hosted in supra-subduction zone (SSZ) ophiolite accounts for a substantial proportion of the global chromium supply market. However, there is no consensus regarding the specific processes involved in the source and formation of this chromium. It seems unlikely that fractional crystallization of basaltic melt or the boninitic melt–mantle harzburgite reaction could provide such huge amounts of chromium given the constraints of Cr mass balance. Here we identify two specific melts responsible for the formation of the typical ophiolite-related Luobusa chromite deposit in the Yarlung–Zangbo Suture Zone in Tibet, China. One is Cr-rich melt derived from the deep asthenosphere, and the other is boninitic melt generated by hydrous melting of previously depleted peridotites. We propose that the Luobusa podiform chromitite was produced through mixing of these two melts, of which the primitive asthenospheric Cr-rich melt provided huge amounts of Cr, and the introduction of boninitic magma triggered the crystallization of chromite. The findings of this study are important for understanding the genesis of global podiform chromite deposits hosted in SSZ ophiolite.

Geochemistry and Geochronology of the Neoproterozoic Backarc Basin Khzama Ophiolite (Anti-Atlas Mountains, Morocco): Tectonomagmatic Implications

The Khzama ophiolite is a highly dismembered complex located in the Siroua inlier of the Moroccan Anti-Atlas Belt. It consists of ultramafic rocks, cumulate gabbros, sheeted dikes, pillow lavas, and an overlying volcano-sedimentary sequence. Three main tectonic slices of sheeted dike complexes are studied in detail along three rivers, exposing well preserved outcrops where individual dikes are clearly distinguishable from the intruded host rock (Assif n’Tinzla, Assif n’Tasriwine, and Assif n’Iriri). Sheeted dikes of the Khzama ophiolitic complex are basaltic to andesitic in composition, displaying a clear sub-alkaline nature. We identify two sets of dikes that originate from lower High-Ti series (HTS) lavas and overlying upper Low-Ti series (LTS) lava. The immobile trace-element signatures of these rocks point to a genesis on a backarc environment with magmas sourced in a supra-subduction zone (SSZ) at the spinel peridotite zone. The obtained SHRIMP U-Pb data of the gabbro represent the first radiometric age of zircon extracted from the mafic rocks that were intruded by the sheeted dike complex of the Khzama ophiolite. These grains yield a concordia age of 763 ± 5 Ma, which is consistent with the 761.1 + 1.9/−1.6 and 762 + 1/−2 Ma U-Pb zircon ages of plagiogranites of Siroua. Based on their mineralogy, modal proportions, and major element chemistry, the felsic dikes are classified as high silica–low alumina trondhjemites or plagiogranites. These plagiogranites were likely formed by the partial melting of mafic rocks rather than by extreme fractional crystallization. A plagiogranite dated at 777 ± 4.7 Ma (U-Pb on zircon) is significantly older than the ca. 762 Ma plagiogranites previously recorded for the Khzama locality, suggesting a long-lived supra-subduction zone (SSZ) with conditions for the hydrous melting of mafic rocks.

Water, Hydrous Melting, and Teleseismic Signature of the Mantle Transition Zone

Recent geophysical and petrological observations indicate the presence of water and hydrous melts in and around the mantle transition zone (MTZ), for example, prominent low-velocity zones detected by seismological methods. Experimental data and computational predictions describe the influence of water on elastic properties of mantle minerals. Using thermodynamic relationships and published databases, we calculated seismic velocities and densities of mantle rocks in and around the MTZ in the presence of water for a plausible range of mantle potential temperatures. We then computed synthetic receiver functions to explore the influence of different water distribution patterns on the teleseismic signature. The results may improve our understanding and interpretation of seismic observations of the MTZ.

A Silicocarbonatitic Melt and Spinel-Bearing Dunite of Crustal Origin at the Parker Phlogopite Mine, Notre-Dame-du-Laus, Quebec, Canada

The Parker phlogopite mine, located near Notre-Dame-du-Laus, Quebec, 74 km north of Ottawa, is well known among mineral collectors for its centimetric euhedral crystals of black spinel. Among the dozens of phlogopite mines active in the early 1900s in the Mont-Laurier–Bancroft corridor in the Central Metasedimentary Belt of the Grenville Province, the Parker mine is exceptional because of the association of forsterite + spinel with phlogopite. Euhedral crystals of these minerals are found “frozen” in a carbonate matrix. The carbonate dike and segregations are associated with spinel-rich dunite that contains accessory diopside, phlogopite, and pargasite, as well as ilmenite and apatite. The interstitial melt crystallized to calcite + dolomite. Hematite appeared as flakes in the melt owing to net loss of hydrogen, and the spinel underwent oxidation-induced exsolution. Our spinel crystal entrapped a domain of carbonate during growth. It also entrapped globules of boundary-layer melt that crystallized to a carbonate + sulfate + phosphate + silicate + oxide assemblage. Such globules, where present in the cumulate, are more pristine than in the coarse crystal of spinel, i.e., less affected by a hydrothermal overprint. We contend that the carbonate melt ultimately formed by the hydrous melting of marble, as supported by oxygen-isotope data on all major minerals. Melting occurred 1140 million years ago, at a time of tectonic relaxation following the Shawinigan compressive stresses.